The monitoring of the level of glucose in blood is important to the management of diabetes. The level of glucose in the blood is controlled by the amount of carbohydrate ingested and by insulin. Too much insulin lowers the glucose level and too little will result in an abnormally high level of glucose. Both circumstances lead to serious health problems for the diabetic.
Most of the glucose testing done outside of the hospital laboratory is done in non-laboratory settings such as nurses' stations, physician's offices and at home. Testing is frequently done by measuring the amount of glucose in urine. As the level of glucose rises in the blood it exceeds the ability of the kidney to reasorb and glucose is excreted into the urine.
Although measurement of glucose in urine is useful, measurement of glucose in blood provides a more accurate reflection of the condition of the subject. Urine glucose does not accurately reflect the level of glucose in the blood since the level of glucose in urine is determined by the level of glucose in the blood and the ability of the kidney to reabsorb the glucose. Therefore, the urine sample cannot tell the diabetic how low his glucose level is.
Dry reagent test strips are widely used for detecting glucose in urine and blood. In general, such test strips comprise plastic strips provided at one end thereof with an absorbent paper portion impregnated with an enzyme system and a color indicator compound which changes color when oxidized. The change in color can be measured by comparing the color formed on the strip with a standard color chart calibrated to various glucose concentrations. More recently, however, to more accurately control the level of glucose in blood, instruments have been developed to measure the color change in a reflectance photometer and thereby give quantitative results.
Although aqueous glucose control samples are provided with these test systems, it is recognized that the controls are not adequate. This is because the values of the aqueous controls do not correspond to the actual glucose control due to factors, principally red blood cells, which are absent in the aqueous controls.
This problem is best seen by making a solution of glucose in water and determining the true value of glucose vs. the value obtained with commercially available glucose test strips. For example:
______________________________________ True Measured Value Value ______________________________________ Chemstrip bG .RTM. (1) 80 mg/dL = 115 mg/dL Dextrostix .RTM. (2) 80 mg/Dl = 149 mg/dL ______________________________________ (1) A trademark of BioDynamic/Boehringer Mannheim Diagnostics, Inc. (2) A trademark of Ames Division of Miles Lab.
Thus, a definite need exists for a stable quality control system for the glucose meter. This need is discussed in two recent articles: von Schenck H, Lonnstrom L and Engstrom M; Quality control of reflectometric determinations of glucose in dried blood spots on filter paper; Clin Chem 31(5):706, 1985; and Burrin JM, Williams DRR and Price CP. Performance of a quality-assessment scheme for blood glucose meters in general practice. Ann Clin Biochem 22:148, 1985. After stressing the poor performance of the strips and the need for a quality control system, both articles suggest the use of filter paper discs containing boric acid to absorb the blood. The blood is then sent to the clinical laboratory for assay. However, the method proposed by the articles has proved ineffective because quality control assessment occurs several days after the test is performed and, therefore, the information is of limited usefulness. The method is also expensive because of the separate assay required.
Similar elevated values obtained with strips are seen for other levels of glucose. The reason for the lack of agreement between the real value and the determined value is not known for certain but it is probably due to the more rapid penetration of the aqueous glucose control solution into the strip as compared to that with blood. In any case, presently available aqueous solutions of glucose are not acceptable controls for the test strips.
A further problem is that the validity of the aqueous glucose control solution cannot be verified by using standard clinical laboratory methods of analysis commonly employed in the hospital (standard methods of analysis give the actual value).
Accordingly, it is the object of the invention to provide a stable glucose reference control with glucose test strips, in which the true value of glucose and the measured value of glucose determined colorimetrically with glucose test strips is approximately the same.